In systems of this nature, the echo profile typically contains a number of false echoes arising from obstacles, reflections and the like. It is well known to provide signal processing which discriminates between these and the true measurement echo. Many systems make use of a time-varying threshold (TVT). The echo of the material sometimes dips below the TVT causing the device to select another echo and potentially go into loss of echo state. There are also instances where an unwanted echo rises above the TVT, causing the device to lock onto that echo and produce incorrect readings.
Up to now, the echo has been tracked by a simple discrimination window: the next echo must fall within the window to be accepted. The window is centred on the position of the current echo, and has its width adjusted based on historical knowledge. The window is initially narrow, and an echo within the window is selected. When no echoes are found within the present window, the size of the window is increased until an echo falls within it.
Unwanted echoes are eliminated from the selection process by the TVT curve. At commissioning, the TVT is shaped around the known fixed obstacles. Once the commissioning is complete, the device no longer has any means to discriminate for obstacles. If the level of an echo from an obstacle rises above the TVT, then the device will lock onto it when the level of the material is below the level of the obstacle. This can be a significant problem, which the present invention seeks to overcome or mitigate.